GB2266777A

GB2266777A - Measuring dielectric constants

Info

Publication number: GB2266777A
Application number: GB9309332A
Authority: GB
Inventors: Nobutaka Nakamura; Masafumi Take; Nobuo Iizuka
Original assignee: Seiko Instruments Inc
Current assignee: Seiko Instruments Inc
Priority date: 1992-05-07
Filing date: 1993-05-06
Publication date: 1993-11-10
Anticipated expiration: 2013-05-06
Also published as: JP3047141B2; US5389884A; JPH05312756A; GB2266777B; GB9309332D0

Abstract

Apparatus for measuring the dielectric constant of a sample (8) comprises a retaining and separating means (9, 10) arranged to receive and retain the sample (8), and to separate it from a pair of electrodes (2, 3). The sample may enter a fluid state as the result of heating (7). The retaining and separating means shown comprises a retaining ring (9) and a pair of separating plates (10). <IMAGE>

Description

APPARATUS FOR MEASURING A DIELECTRIC CONSTANT The present invention relates to an apparatus for measuring a dielectric constant of a sample.

It is known to measure the dielectric characteristic of a material as a function of temperature, frequency, or time and it is known to evaluate the material in terms of electrical, chemical, or physical characteristics. The measurement generally is performed by arranging a sample between a pair of parallel plate electrodes and measuring the capacitance produced between the electrodes.The dielectric constant is obtained by applying the measured capacitance value to the following expression: C = (S/d) s whereby C: Capacitance S : Electrode area d: Distance between electrodes (sample thickness) s: : Relative dielectric constant of the sample 80 : Dielectric constant of vacuum Thus the dielectric constant of a sample can be measured by using electrodes of a known area and measuring the distance between the electrodes, or the thickness of the sample.

However, measuring the dielectric constant at various temperatures can involve side effects such as: the sample changing state (eg. becoming a fluid), a change in the thickness of the sample through cooling, and welding between the electrodes and the sample. Such effects result in lowered measuring accuracy as well as experimental inconvenience. Effective countermeasures against such problems have been proposed as follows: (1) An interdigital activating electrode and an response electrode are arranged on a ceramic substrate so as to be close to each other. (Micromet, Umetric, System II Option S-60 type ceramic sensor manufactured by Micromet Co. in U.S.A.) (2) A sample sandwiched between insulating thin film members is measured using parallel plate electrodes.

(3) Parallel plate electrodes are formed on a ceramic substrate so as to be disposable should the sample become welded to the electrodes. The distance between the electrodes is measured and controlled. (Refer to Japanese patent application laid-open number 85770-1990.) The above proposed methods have the following disadvantages: Method (1) cannot provide a spatially uniform electric field, as compared with the method using parallel plate electrodes. Since the sample is arranged in an uneven electric field, the measured values depend disadvantageously on the shape of the sample.

Furthermore since the electrodes are formed on a single ceramic substrate, the measurement result involves a dielectric constant component of the ceramic substrate material. This lowers the accuracy of the measurement.

Method (2) can prevent contamination of the electrodes due to welding which might arise if a sample in a fluid state is directly in contact with the electrodes.

However, it is difficult to maintain constant the distance between the parallel plate electrodes during the measurement. Particularly since the structure used in method (2) cannot prevent a sample from spreading in the radial direction, it is difficult to maintain the sample in a fixed shape during the measurement. Furthermore, the dielectric constant of the insulating material inserted between the sample and an electrode may cause a measuring error in the measured dielectric constant of the sample.

In method (3), the flatness of the electrode manufactured on the ceramic substrate tends to be of an inferior quality compared with that of an electrode manufactured using conventional metal processing. In addition, when the ambient temperature of the electrodes is varied over several hundreds CO, it is extremely difficult to measure the distance between the electrodes to an accuracy of within several Ktm . Hence, there is the disadvantage that the method is impractical.

The present invention seeks to overcome the above mentioned disadvantages and particularly to improve upon method (2).

According to the present invention there is provided an apparatus for measuring a dielectric constant of a sample, comprising a pair of electrodes and a retaining and separating means, the retaining and separating means being arranged to receive the sample so as to retain the sample and separate it from the electrodes.

Preferably, the retaining and separating means comprises a pair of separating members and a retaining member, the retaining member being arranged to receive the sample so as to retain the sample between the separating members which separate the sample from the electrodes.

Beneficially, the apparatus includes a heating oven arranged for heating of the sample.

It is preferred that the pair of electrodes consist of an activating electrode and a response electrode and further including a ground electrode and a circuit for detecting the conducting current on the response electrode.

Advantageously, the response electrode is separated from and supported by the ground electrode via a spacer.

An embodiment of the invention will now be described by way of example only and with reference to the accompanying drawing, in which: Figure 1 is a cross sectional view, partially including block diagrams, showing a preferred embodiment according to the present invention.

In an embodiment according to the present invention the complex dielectric constants of the separating members (insulating thin film members) and the retaining member (insulating ring member) are as follows: The complex dielectric constant of the insulating thin film members is: 8L* = SL - i SL and the complex dielectric constant of the insulating ring member is: #R* = #R' - i #R" The insulating thin film members and the insulating ring member are each made of a discrete and standard disk form. They are subjected to a 'pre-use' measurement in order to examine the dependence on temperature and frequency. Next, the insulating ring member is arranged on an insulating thin film.Then a sample to be measured is applied, so as to fill the ring, and is covered with the other insulating thin film. The sample and the ring are held between the insulating thin films which are themselves sandwiched between the parallel plate electrodes, in a similar manner to a convention sample (ie. a sample sandwiched between insulating thin film members - without the ring). The entire structure is subjected to a measurement of the complex dielectric constant with frequency and/or temperature being varied.Then the relationships between the capacitance values and dielectric constants of: the sample, the insulating thin films, the insulating ring and the composite capacitance value and dielectric constant of the sandwiched sample, are calculated from the following expressions: Capacitance 1/CM* = 1/(C* + CR*) + 2/CL* .... (1) whereby C*: Complex capacitance of the sample CR*: Complex capacitance of the insulating ring CL*: Complex capacitance of the insulating thin films CM*: Complex capacitance of the sandwiched sample The Relation Between Capacitance and Relative Dielectric Constant C* = (SR/tR) #O #* .... (2) CR* = ((S - SR) / tR) #O 8R* .... (3) CL* = (S / tL) SO SL (4) CM* = (S/tM) #O #M* (5) where:- tR : Thickness of insulating ring tL: :Thickness of insulating thin film tM: Thickness of the sandwiched sample (= tR + 2tl) S : Electrode area SR: Inner area of the insulating ring #O: Dielectric constant of vacuum #*: Complex relative dielectric constant of the sample (= #' - i s") #R*: Complex relative dielectric constant of the insulating ring (= #R' - i #R") 8L*: Complex relative dielectric constant of the insulating thin film (= #L' - i#L") 8M*:: Measured complex relative dielectric constant of the sandwiched sample (= sM - i 8M") The complex relative dielectric constant s* (= s' - i s") of the sample being measured can be expressed using SR*, 8L* and #M* at the same temperature and frequency. By substituting expressions (2) to (5) for expression (1) to solve s*, the following expression is given: #'=

.... Expression (6)

....Expression (7) The dielectric constant of the sample itself can be measured by outputting as the dielectric constant of the sample the complex relative dielectric constant 8M* (= s' - i s") based on expressions (6) and (7).

In this case, materials (for instance, polytetrafluoroethylene, alumina) which are not softened within the measuring temperatures may be used for the insulating thin films and insulating ring, whereby a measurement can be performed accurately while a sample in a fluid state is maintained within the ring.

A detailed explanation is given below for an embodiment according to the present invention, with reference to figure 1.

In figure 1, numeral 1 represents a function generator for generating a sine wave voltage having a predetermined amplitude and frequency. The generator 1 provides a voltage signal to a terminal 2a of an activating electrode 2. Electrode 2 has a disc portion which produces an electric filed across a sample. A response electrode 3 having a flat surface facing the flat surface of the activating electrode 2, is arranged under the activating electrode 2 and is supported on a ground electrode 5 by way of alumina ring spacer 4. The activating electrode 2, the response electrode 3, and the ground electrode 5 are all made of stainless steel, and are arranged to form a three-terminal parallel plate electrode. The sample is generally positioned between the activating electrode 2, and the response and ground electrodes.A current detector 6 is connected to a terminal 3a of the response electrode 3. Detector 6 is also connected to a terminal 5a of the ground electrode 5, and is thus capable of detecting the conducting current on the response electrode 3.

Reference numeral 7 represents a heating oven. The heating oven 7 is arranged to surround the electrodes and the sample so as to be able to vary the temperature of the sample.

The sample 8 is inserted in a ring 9, formed of Teflon, and is sandwiched between two laminas 10, also formed of Teflon. The arrangement is beneficial because it is otherwise difficult to remove the sample from between the electrodes when the sample is a viscous fluid or is predicted to change to a fluid state at varying temperature.

The sandwiched sample (comprising the sample 8, the ring 9, and the two laminas 10) is inserted between the activating electrode 2, and the response and ground electrodes 3 and 5 to enable a dielectric constant of the sample to be measured. The temperature of the sample is measured with a thermocouple 11 inserted in a recess formed in the lower portion of ground electrode 5.

Activating electrode 2 is screwed to an alumina supporting plate 14 which is fixed on a supporting strut 13 erected on a base 12. Ground electrode 5 is held on an alumina sleeve 15 supported on a stainless steel member 16. Member 16 has a generally cylindrical shape with a handle 16a adjacent it's upper surface. The handle 16a is used to assist in placing the sample between the electrodes, or removing it therefrom.

Member 16 is received in a hollow cylinder 17 which has substantially the same inner diameter as the outer diameter of member 16. Cylinder 17 is fixed on base 12. A spring 18 is arranged in a compressed state, partially received in member 16 and acting against base 12. Spring 18 pushes upwardly on the ground and response electrodes, via components 16 and 15. During the measurement operation, the sample is pressed against electrode 2 at a constant pressure.

A detent 16b protrudes from the side wall of member 16 and engages with a Tshaped cutaway 17a formed in the cylinder 17. In positioning or removing the sample, an operator can latch securely the detent 16a in cutaway portion 17, by pushing down and twisting the handle 16a. This locks electrodes 3 and 5 in an 'open' position whereby the sample can be positioned or removed.

Heating oven 7 is lowered and raised by a mechanism 18 which attaches the oven to the based 12 by a vertical shaft.

A complex dielectric constant measuring circuit 19, which performs a wellknown measuring method, receives signals from function generator 1 and current detector 6. Circuit 19 outputs the measured value of the complex dielectric constant.

The measured value is stored in a memory 20. An operational circuit 21, connected to the memory 20, calculates the complex dielectric constant of the sample 8 according to the following expression:

where: s": Complex dielectric constant real part (Storage dielectric constant) s": Complex dielectric constant imaginary part (Loss dielectric constant) t: Thickness S : Electrode area SR: Inner area of the ring M: Subscript representing the measured sandwiched sample L: Subscript representing the lamina R: Subscript representing the ring Furthermore the loss tangent tan 6 can be easily obtained by applying both the storage dielectric constant s' and the loss dielectric constant " to the following expression: tan 3 = s" /' Namely, according to the present invention, a change in the dielectric constant at or during a softening process of a viscous fluid sample (for instance, a hardening process of an adhesive agent) can be measured without direct contact of the sample with the electrodes.

In the above explanation, the retaining and separating means have been described in terms of a retaining member, in the form of a teflon insulating ring, and a pair of separating members, in the form of teflon insulating thin films. However, the retaining and separating means may be in the form of a teflon dish with a teflon lid, and other such variations will be readily apparent. Similarly, stainless steel has been recited as the electrode material but other good conductive metal materials may be used. The surface of the conductive metal may, for example, be plated with gold. When a lamina or ring material having an increased heat resistance is required, sinter alumina or other insulating material may be used.

As described above, according to the present invention, since a change in dielectric constant (with respect to temperature, frequency, and/or time) of a fluid sample can be measured without direct contact between the sample and the electrodes, the sample can be positioned and removed very easily. During the measurement operation, the deformation of the sample due to softening or hardening can be suppressed because of the presence of the lamina and ring. Hence, sample deformation is mitigated, so that the reliability of the measured value can be increased effectively. Furthermore, since there is no severe restriction upon the electrode material, the flatness of the stainless steel electrode can be increased to a maximum extent. There is no contamination due to sample welding and, accordingly, expensive electrodes can be used effectively without risk of damage.

Claims

1. An apparatus for measuring a dielectric constant of a sample, comprising a pair of electrodes and a retaining and separating means, the retaining and separating means being arranged to receive the sample so as to retain the sample and separate it from the electrodes.

2. An apparatus as claimed in claim 1, wherein the retaining and separating means comprises a pair of separating members and a retaining member, the retaining member being arranged to receive the sample so as to retain the sample between the separating members which separate the sample from the electrodes.

3. An apparatus as claimed in claim 1 or 2, further including a heating oven arranged for heating of the sample.

4. An apparatus as claimed in any preceding claim, wherein the pair of electrodes consist of an activating electrode and a response electrode and further including a ground electrode and a circuit for detecting the conducting current on the response electrode.

5. An apparatus for measuring a dielectric constant of a sample substantially as hereinbefore described with reference to and as illustrated in the accompanying drawing.

5. An apparatus as claimed in claim 4, wherein the response electrode is separated from and supported by the ground electrode via a spacer.

6. An apparatus for measuring a dielectric constant of a sample substantially as hereinbefore described with reference to and as illustrated in the accompanying drawing.

Amendments to the claims have been filed as follows 1. An apparatus for measuring a dielectric constant of a sample, comprising a pair of electrodes and a retaining and separating means having a pair of separating members and a retaining member, the retaining member being arranged to receive the sample so as to retain the sample between the separating members which separate the sample from the electrodes.

2. An apparatus as claimed in claim 1, further including a heating oven arranged for heating of the sample.

3. An apparatus as claimed in any preceding claim, wherein the pair of electrodes consist of an activating electrode and a response electrode and further including a ground electrode and a circuit for detecting the conducting current on the response electrode.

4. An apparatus as claimed in claim 3, wherein the response electrode is separated from and supported by the ground electrode via a spacer.